The invention pertains to the field of nonlinear optical frequency mixing processes and more particularly to an apparatus for servo-locking a nonlinear crystal to the angle at which phasematching occurs.
The use of nonlinear optical frequency mixing processes such as second harmonic generation has an increasingly important role in applications of lasers in commercial and research environments. The generation of radiation at maximum output requires alignment of the optical axis of birefringent crystals at the proper phasematching angle with respect to the direction of propagation of incident laser beams. The phasematching angle is a function of laser frequency and also temperature. This causes the intensity of the radiation which results from the nonlinear mixing process in the crystal to drop off sharply if the frequency of the incident beam varies, if the crystal is rotated slightly from the proper phasematching angle, or if the crystal heats up upon the passage of the laser beams therethrough or due to room temperature fluctuations. In order to return to maximal output, the realignment of the crystal with respect to the beam becomes necessary. The problem becomes amplified in laser systems utilizing many nonlinear optical frequency mixing processes or for systems that need to vary laser frequencies.